The drilling of wells for oil and gas production conventionally employs relatively small diameter drilling pipe joined end to end to form a drill string to which is secured the necessary equipment including a drill bit for creating a wellbore which is of larger diameter than the drilling pipe. After a portion of the wellbore has been drilled, the wellbore is usually lined with a string of tubular casing member joined end to end to define a casing string. This conventional approach requires a cycle of drilling the wellbore, pulling the drill string out of the wellbore to the surface and running casing into the wellbore. The process is time consuming and costly.
The technique of casing drilling has been developed to address the problems of conventional drilling. The casing drilling process involves running a casing string into the wellbore with the drilling string.
Using either of the above techniques, a wellbore may be drilled and then cased to a certain depth, and then the drilling apparatus removed. If the depth of the wellbore is ever later to be extended, it is not possible to reinsert the drilling apparatus into the cased wellbore without resorting to a smaller diameter casing string. As different lower segments of the wellbore are drilled, successively smaller diameter casing strings are required in order to pass through the casing strings above.